The oil sands deposits of northern Alberta in Canada contain about 142 billion cubic meters (or 890 billion barrels) of bitumen, thus constituting the largest oil sands deposit in the world. In the Athabasca region of Alberta, the oil sands deposits are typically composed of (by weight) about 12% bitumen, 82% to 85% mineral matter (solids), and 3% to 6% water. Of the solids fraction, the solids smaller than 45 microns in size (i.e., silt and clay) are referred to as fines. The clay fraction of the fines can be a significant factor in processes for both extraction of bitumen and disposal of oil sands tailings (i.e., residue from primary oil sands processing).
The bitumen content of the Athabasca deposits has been commercially utilized by oil sands ore-water slurry-based extraction processes and thermal in-situ processes, and upgraded to synthetic crude oil at a production capacity of over one million barrels per day. In the major bitumen recovery operations in the Athabasca region, bitumen is produced from surface-mineable oil sands using water-slurry-based extraction processes, in which the oil sands “ore” (i.e., the raw oil sands material, as excavated from the oil sands deposits) is mixed with hot water to form an ore-water slurry. Asphaltic acids present in bitumen, which contain partly aromatic oxygen functional groups such as phenolic, carboxylic and sulphonic types, become water-soluble, especially when the ore-water slurry's pH (i.e., a measure of acidity expressed as the minus logarithm of the hydrogen ions concentration: pH=−log [H+]) is slightly over 7, and act as surfactants reducing surface and interfacial tensions. Reduction of the surface and interfacial tensions of the ore-water slurry system results in the disintegration of the ore structure and liberation of bitumen from the ore. Accordingly, the water-soluble fraction of bitumen asphaltenes plays an important role in the recovery of bitumen from the surface-mineable oil sands ore.
It is known that the water-soluble fraction of bitumen is increased by an increase in the pH of an oil sands ore-water slurry by the addition of caustic soda (NaOH), soda ash (Na2CO3), or any salt of weak acid and strong base (hydrolysis of which would be basic). Alternatively, this desirable result can be achieved by modifying the asphaltene molecules contained in bitumen by oxidation, sulfonation, and/or sulfoxidation reactions to form water soluble surfactants, which reduce the surface and interfacial tensions: see International Application No. PCT/CA2005/001875 (WIPO Pub. No. WO 2006/060917).
Liberated bitumen has to be recovered from the ore-water slurry by separation methods based on density differences. Bitumen density is very close to the density of water; consequently, bitumen needs to become effectively “attached” to air bubbles in order for it to be recovered from the ore-water slurry system by means of flotation. However, clay particles present in the ore can become attached to bitumen droplets, thus preventing the desired interaction between bitumen droplets and air bubbles. This undesirable attachment of clay particles to bitumen is promoted by calcium (Ca2+) and magnesium (Mg2+) ions present in the process water. Another significant factor with respect to bitumen recovery is that the temperature of the ore-water slurry has to be above a critical temperature, above which bitumen becomes mobile enough (i.e., has sufficiently low viscosity) to enfold air bubbles and thus facilitate flotation. This critical temperature for Athabasca bitumen is reported by several researchers to be around 32° C.
In summary, liberation of bitumen from the oil sands matrix and attachment of air bubbles to the liberated bitumen are essential process steps for bitumen recovery in ore-water slurry-based extraction processes.